1. Field of the Invention
This invention relates in general to liquid crystal display devices, and more particularly, to bistable twisted-nematic liquid crystal devices and a method of driving such display devices.
2. Discussion of the Background
A great deal of emphasis has been placed in recent years on liquid crystal display devices. With recognized advantages such as low electrical power consumption and small size, liquid crystal display devices are widely used for audio equipment, instrument panels, office automation equipment, and other uses.
Liquid crystals, which include ordered molecules or groups of molecules in a liquid state, are considerably useful for fabricating these devices for switching, modulating, and otherwise altering the characteristics of light beams. Both differences in transmittance and in the polarizing effect of such liquid crystals have been now utilized for fabricating these devices.
However, it would be more practical for a number of new applications to have a liquid crystal material which has two stable states, and which can be easily transformed from one stable state to the other, rapidly and with a minimum expenditure of energy.
To implement a high speed drive of liquid crystal devices, a variety of liquid crystal displays using bistable twisted-nematic (BTN) liquid crystals have been disclosed as exemplified in Japanese Published Patent Application No. 1-51818 and Japanese Laid-Open Patent Applications Nos. 6-230751, 8-101371 and 8-313878.
Bistable characteristics are shown for twisted-nematic liquid crystals in these disclosures, in which at least two pulse voltages are applied to produce an electric field across a liquid crystal cell. A first pulse is used to initiate the Freedricksz transition of the liquid crystal and a second pulse is used to subsequently relax the liquid crystal to either one of two metastable states, thereby modulating optical transmittance or reflectivity to be utilized as display devices.
Although principles for switching behavior of possible displays are presented in JPA 1-51818, no description is made of driving the displays. Also, JPA 6-230751, 8-101371 and 8-313878 propose basics of driving simple matrix type displays. However, no description is made of either the effects of temperature on display quality, or methods of compensating for the effects, which is deemed important for practical purposes for the display devices.